I do something like this, except I use keyboard maestro to capture changes to the price. The only problem is I have to actually visit the page for KM to capture it, or, well, I have an applescript that occasionally opens the page, checks, and closes it, but that also has the problem of only working when my computer is on. Your solution sounds much smoother!
But anyway, yeah, automation for the win. I'm totally with you there.
I hear Keyboard Maestro is nice. I got a license in some Mac bundle but I don’t use Mac much outside of my laptop. Used to when I was making Apple Store games.
Check out Change Detection it is great and self hostable and can do full browser emulation.